Photos:: Democratic presidential candidates debate at the University of Wisconsin-Milwaukee

Democratic presidential candidates Hillary Clinton and Bernie Sanders participate in the PBS NewsHour Presidential Primary Debate in Milwaukee, Wis.

Democratic presidential candidate Hillary Clinton participates in the PBS NewsHour Presidential Primary Debate with Bernie Sanders in Milwaukee, Wis.

Presidential candidate Sen. Bernie Sanders participates in the PBS NewsHour Democratic debate at the University of Wisconsin-Milwaukee.

Democratic presidential candidates Bernie Sanders and Hillary Clinton take the stage for their debate at the University of Wisconsin-Milwaukee in Wisconsin.

The candidates during the early moments of their debate in Milwaukee.

Bernie Sanders addresses a question.

Hillary Clinton during the debate.

Bernie Sanders and Hillary Clinton square off in the debate, hosted by PBS.

Hillary Clinton supporters gather outside the debate in Milwaukee.

A Bernie Sanders supporter on debate night.